Blindness is often the result of degeneration of the light sensitive rods and cones of the retina in conditions such as retinitis pigmentosa and macula degeneration. An implantable vision prosthesis has been developed that electrically stimulates the retinal ganglion cells in such a way that action potentials in retinal ganglion cells are evoked, causing a visual sensation in the visual cortex. Clinical trials are about to be conducted.
W. Mokwa RWTH Aachen University, Aachen, Germany
The concept
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Retinal degeneration is mostly caused by macular degeneration and retinitis pigmentosa. In retinitis pigmentosa the degeneration process, that is, the dying of the light sensitive rods and cones, starts from the outer region of the retina. Gradually, the field of sight becomes smaller and the degeneration results in total blindness after a long period.
The retinal ganglion cells are responsible for preprocessing the visual information. As many as 30% of the retinal ganglion cells of patients with retinitis pigmentosa can survive even after several years of blindness. It has been shown that electrical stimulation of these ganglion cells yields visual sensations.1,2,3 Therefore it seems possible to bypass the degenerated photoreceptors by electrical stimulation of retinal ganglion cells and is system based on this concept and comparable with a cochlea implant seems to be feasible.4
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Figure 1: Schematic view of the system. |
There are two approaches to the stimulation of retinal ganglion cells. In the subretinal approach stimulation electrodes are placed behind the retina at the place of the degenerated rods and cones.5 In the epiretinal approach the stimulation electrodes are placed on the retina from inside the eye.6 This article focuses on the epiretinal approach and describes the work performed by a German research group under an EPIRET 2 grant, which is funded by the German Ministry for Education and Research.
The epiretinal implant system
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Figure 2: View through the polyimide film onto the receiver chip. |
Figure 1 shows a schematic view of the system. The system consists of an extraocular part and an intraocular part. In the extraocular part a camera takes visual images. Using a digital signal processor, the visual images are transformed via a neural net into control data for the stimulation part of the system.4 These data are finally transmitted into the interior of the eye via a radio frequency (RF) link, together with the energy needed to supply the intraocular part. On the implant, the data are decoded in a receiver unit and transferred via connectors to the stimulation part of the system.4
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Figure 3: Photo of an encapsulated retina implant system. The left part of the picture shows the receiving part encapsulated into an artificial lens. The right part of the picture shows the encapsulated stimulator chip and the stimulation electrodes. |
The receiver unit consists of a small coil, a receiver chip for power and data recovery (Figure 2) and some surface mount devices. Metal lines connect the receiving unit to the stimulation unit. The stimulator consists of a stimulator chip and planar stimulation electrodes made from platinum. All components are assembled onto a 10 µm thin polyimide foil. The receiver unit is located in an artificial lens. Because a wireless connection RF coupling is implemented using resonator circuits, the data, which have been coded to guarantee a safe transmission, are separated from the carrier by demodulation and decoding. The electrical power is extracted from the RF signal by rectifying this voltage. The data are forwarded via an integrated micro cable in a serial manner to the stimulator chip. The die area of the receiver chip is approximately 5 mm2. The current consumption is approximately 170 µA. The coded data rate amounts to 200 kbit/s. Both receiver and stimulator chip are driven with the same 10-V power supply extracted from the RF signal.
According to the received data, the stimulator circuitry (die area approximately 3 mm2) selects the stimulation electrode and delivers bipolar stimulation pulses to it. This circuitry can drive up to 25 electrodes. The pulses are adjustable to pulse widths from 10–1130 µs and pulse currents from 0–100 µA. The current consumption at maximum stimulation current amounts to 330 µA. A pulse rate of up to 500 Hz is possible.
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Figure 4: Data stream to the stimulator chip (velvet) and a bipolar stimulation pulse (red) measured at the stimulation electrodes (courtesy of FHG IMS Duisburg, Germany). |
After assembly of the different parts, the device is encapsulated with biocompatible parylene and silicone to prevent eye liquids from reaching the electronic components and to ensure biocompatibility of the whole system. Figure 3 shows a fully encapsulated system. Figure 4 shows the serial data stream to the stimulator chip (violet) and the bipolar stimulation pulses (red) directly measured by micro needles from the stimulation electrodes. These systems have been successfully implanted into animals. It has been shown that after stimulation of ganglion cells by the implant system activity in the visual cortex of the brain could be measured.6
Improvement of the retina implant system
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Figure 5: Miniaturised implant before encapsulation. On the left is the planar coil. The electronic components are now on the connetor to the stimulation electrodes. |
It should be pointed out that with 25 electrodes only the visibility of the system can be shown. A product would require a higher number of stimulation electrodes (more than 200) to give a patient at least some limited vision of his/her surroundings. Therefore an improved further miniaturised version of the implant system was developed (Figure 5). The coil for this system is planar and fabricated by micro electroplating of gold directly onto the polyimide foil. For assembly of the receiver and stimulator chip, flip chip technologies were used. To obtain a better coupling to the ganglion cells, three-dimensional electrodes were fabricated by micro electroplating of gold. The gold surfaces were then covered with iridium oxide to render them capable of high charge delivery.7
Outlook
After successfully testing the improved epiretinal implants in animals, the next step is to implant the systems for approximately one month into the human eye of patients suffering from retinitis pigmentosa. These tests are about to be performed. Only with the help of these patients will it be possible to find out whether restoration of vision will in principle be possible with these systems. Should these tests be successful, a further development of a system with a higher number of electrodes will be necessary to produce a viable product.
Acknowledgement
This work is supported by the German Ministry of Education and Science (BMBF). The author would also like to mention the other members of the German EPIRET 2 Team: Institute of Neuroinformatics (University of Bonn), Institute of Biophysics (University of Marburg), Department of Optoelectronics (University of Duisburg), Institute of Pathology (Aachen University), Department of Ophthalmology (University of Cologne), Department of Ophthamology (University of Essen), Fraunhofer Institute of Biomedical Engineering (IBMT) (St. Ingbert), and Fraunhofer Institute of Microelectronic Circuits and Systems (IMS) (Duisburg).
References
1. J.L. Wyatt et al, “Development of a Silicon Retinal Implant: Epiretinal Stimulation of Retinal Ganglion Cells in the Rabbit,” Invest. Ophthal.&Vis. Sci., 35 (Suppl.), 1380 (1994).
2. A. Benjamin et al., “Characterisation of Retinal Responses to Electrical Stimulation of Retinal Surface of Rana Catesbeiana,” Invest. Ophthal.&Vis. Sci., 35 (Suppl.), 1832 (1994).
3. M.S Humayun et al., “Visual Perception Elicited by Electrical Stimulation of the Retina in Blind Humans,” Arch. Ophthalmol., 114, 40–46 (1996).
4. M. Schwarz et al., “Single Chip CMOS Imagers and Flexible Micoelectronic Stimulators for a Retina Implant System,” Sensors and Actuators A: Physical, 83, 1–3, 40–46 (2000).
5. E. Zrenner, “Will Retinal Implants Restore Vision?” Science, 295, 5557, 1022–1025 (2002).
6. W. Mokwa, “MEMs Technologies for Epiretinal Stimulation of the Retina,” J. Micromechanics and Microengineering, 14, 12–16 (2004).
7. E. Slavcheva et al., “Sputtered Iridium Oxide Films as Charge Injection Material for Functional Electrostimulation,” J. Electrochem. Soc., 151, 7, E226-E237 (2004).
Professor Wilfried Mokwa is Director of the Institute of Materials in Electrical Engineering I of the RWTH Aachen University, Sommerfeldstrase 24, D-52074 Aachen, Germany, tel. +49 241 802 7810, e-mail: mokwa@iwe1.rwth-aachen.de, www.iwe1.rwth-aachen.de
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